I. Yu. Prokhorov

Stability of structural materials based on ZrO2

The thermal and mechanical stability of some high-strength ceramic materials from partially stabilized ZrO2 manufactured from various domestic and imported powders, including coprecipitated, sol-gel, and hydrothermal ones, with the use of CIP and sintering is considered. The thermal stability is tested under conditions close to the operating ones, i.e., under long-duration holds at 1000 and 1550°C and in water quenching. The mechanical stability is determined in impact-erosion wear and under combined loads of high pressure and multiple indentations by solid particles. It is shown that all the materials undergo degradation of various degrees but those most durable under normal conditions (hydrothermal and sol-gel materials, ceramics manufactured from imported press powders) are least stable. They have widely fluctuating properties under cyclic high-temperature loads, endure 900-1400°C, and withstand a pressure of at most 1.0-2.0 GPa in an abrasive, just like standard corundum ceramics; however, they are characterized by maximum wear resistance. At the same time, an original material from commercial coprecipitated PSZ powder has quite different features; its thermal stability allows it to withstand repeated quenchings from 1550°C in water, and the mechanical strength can attain 2.6-2.8 GPa, exceeding the strength of quenched tool steels in similar situations. Due to its refractoriness (2700°C) and chemical stability this material is the most versatile in operating under extreme conditions.

Zirconia ceramics from coprecipitated powders

The possibility was explored of making dense and strong partly stabilized zirconia (PSZ)-based ceramic materials from coprecipitated zirconium and yttrium hydroxide powders of both factory and laboratory preparation. The effect of dry and wet grinding, powder burning, cold isostatic pressure (CIP) at≤0.8 GPa, and sintering at ≤1600°C on the physicochemical properties of the material was investigated. It was found that the properties (a density of 5.7–5.8 g/cm3, a bending strength of 600 to 800 MPa, and a crack resistance of 7–9 MPa·m1/2) of the resulting ceramic material would not be reproduced unless the tendency of the PSZ powder to agglomerate spontaneously in storage is overcome or avoided. It is shown that in contrast to other similar materials the ceramic material from a deagglomerated powder has a higher optimal CIP pressure (0.6 GPa), which implies that the material has an improved thermal endurance and a better mechanical stability.

Cold isostatic pressing as a method for fabricating high-strength ceramic materials based on ZrO2

High-strength ceramic materials fabricated with the use of cold isostatic pressing (CIP) at ≤0.8 GPa and sintering by different regimes from yttria-stabilized zirconia produced by leading foreign and Ukrainian firms are described. All the materials exhibit two peaks of mechanical properties, one at a low CIP pressure (0.1–0.3 GPa) attributable to destabilization of the press-powder and characterizing the stability of the material as a whole and the other at a high CIP pressure (above 0.6 GPa) attributable to attainment of close packing of the particles. In accordance with the set of standard properties the materials can be classified into two types, namely, intermediate-strength ones with an ultimate bending strength of 700–900 MPa,K1c ranging from 7 to 12 MPa·m1/2, a density of about 6 g/cm2, and an optimum CIP pressure of 0.2–0.3 GPa (the first peak) and high-strength materials with an ultimate bending strength of about 1200 MPa,KIc ranging from 6 to 9 MPa·m1/2, a density of about 6.1 g/cm3, and an optimum CIP pressure of 0.1 GPa. The intermediate type comprises the majority of domestic materials and some foreign ones and is characterized by higher stability and crack resistance. The high-strength type comprises mainly foreign materials with high strength and lower stability.

Effect of Quasi-Hydrostatic Compression on the Mechanical Properties of Ceramics in the ZrO2 + 3 MOL.% Y2O3 System

The effect of quasi-hydrostatic compression on the strength of ZrO2 + 3 mol.% Y2O3 ceramic specimens of two series was studied. The series 1 ceramic was a powder commercially available from TOSOH Co. (Japan), with a density of 6.1 g/cm3, and the series 2 ceramic was a powder with a density of 5.9 g/cm3 prepared under laboratory conditions at the IPM Research Institute (National Academy of Sciences, Ukraine). The pressure range was up to 1.2 GPa, and the pressure-transmitting medium was a coarse-grained corundum powder. In the series 1 specimens, the strength increases with pressure over the entire pressure range (from 670 MPa to 1098 MPa at 1.2 GPa); in the series 2 specimens, the strength increases only to a pressure of 0.8 GPa (from 695 MPa to 828 MPa) and then, with further increase in pressure drops sharply to nearly zero (30 MPa at 1.2 GPa). It was proposed that the observed effect might be associated with a martensite transformation in the zone of structural imperfections (discontinuities). On reaching a critical value determined by the strength of the matrix, the martensite transformation becomes a cause of failure of the material.

Thermal Stability of Oxide-Based Ceramic Materials

A correlation analysis of the thermal stability of oxide-based ceramic materials (including partially yttria-stabilized zirconia (PYSZ), beta-alumina, and titanium dioxide) produced in batches by different technologies is carried out. A generalized criterion for thermal stability is proposed that takes into account partial compensation of the thermally generated elastic strain owing to compliant behavior of the network of surface cracks. The criterion provides a satisfactory description of experimental data, allows prediction of the behavior of materials under severe thermomechanical conditions, and suggests ways toward further improvement of material properties.

Experimental technology for manufacturing ceramic articles

The production of ceramic articles includes five principal operations, namely, preparation of the powder, pressing of the preform, its treatment, sintering, and grinding. If it is necessary to increase the adaptability to manufacture and improve the physicomechanical properties of the sintered material, the operations of hydrostatic treatment of the powder, preliminary sintering of the preforms, repeated mechanical treatment, etc. can be added. The technology is used for manufacturing the working part of cutting tools, dies, tools for shaping alloys of nonferrous metals, bearings, hinges, valves, ball locks, nozzles of hydromonitors, parts of pressing molds, lining parts, milling bodies, etc.

Role of cold isostatic pressing in the formation of the properties of ZrO2-base ceramics obtained from ultradisperse powders

The physicomechanical properties of ceramics obtained from plasmachemical and sol-gel powders of partially stabilized (3% Y2O3) zirconia (PSZ) and its compositions with 20% Al2O3 by cold isostatic pressing (CIP) at a pressure of at most 2 GPa and sintering at 1300–1650°C are investigated. It is established that plasmachemical PSZ exhibits its best properties (Klc=7.8 MPa · m1/2, a strength of 650 MPa) only after complete disintegration at a CIP of 0.1 GPa and a sintering temperature of 1650°C, when the material is sintered to a density of 5.5 g/cm3. After partial stabilization and CIP at 0.1 GPa the plasmachemical composition of PSZ+20% Al2O3 is sintered at 1650°C to a density of 4.7 g/cm3, but hasKlc=8.5 MPa · m1/2 and a strength of 700 MPa. The deagglomerated sol-gel powder exhibits properties at a level ofKlc=12.4 MPa · m1/2 and a strength of 950 MPa at a density above 6.0 g/cm3 after CIP at 0.3 GPa and sintering at 1450°C. The latter obviously has the best mechanical properties of all the investigated materials.

Crack resistance of hydrostatically compressed alkaline halide single crystals

Conclusions1.The effective fracture toughness of the investigated alkali halide single crystals increases with an increase in pressure.2.This effect is described well by Eq. (11), which was derived on the basis of the assumption of the addition to the effective surface energy caused by the action of high hydrostatic pressure on the elastic tip of the crack and of plastic relaxation of the stresses as the basic controlling factors.3.The model obtained predicts the same slope of the KIc*(P) relationship with small values of P for all brittle and semibrittle crystalline bodies in the geometry of the double cantilever beam specimen. In intermediate cases investigations of the dislocation structure are necessary for description of this relationship.4.For LiF single crystals a transition from bittle cleavage to semibrittle failure, which is controlled by the appearance and elongation of the slip lines (dislocation accumulations) originated by the crack, was observed. The dislocation structure in cleavage of NaCl and KCl makes it possible to include these crystals among initially semibrittle bodies.5.One of the main reasons for the change in mechanical behavior of crystals and the increase in dislocation activity under high hydrostatic pressure conditions is the increase in the rate of liberation of elastic energy for accomplishment of the work for formation of the elastic tip of the moving crack.